How bad (or possibly good) value drift and lifestyle drift are will depend your definition of the phenomenon, as you acknowledge yourself. The way I conceptualise them in the EA Forum article I wrote on the topic ('Concrete Ways to Reduce Risks of Value Drift'), makes them strongly net-negative. In the post I (briefly) make the case that reducing risks of value drift and lifestyle drift may be an altruistic top-priority.
Here's how I think about the topic:
I use the terms value drift and lifestyle drift in a broad sense to mean internal or external changes that would lead you to lose most of the expected altruistic value of your life. Value drift is internal; it describes changes to your value system or motivation. Lifestyle drift is external; the term captures changes in your life circumstances leading to difficulties implementing your values. Internally, value drift could occur by ceasing to see helping others as one of your life’s priorities (losing the ‘A’ in EA), or loosing the motivation to work on the highest-priority cause areas or interventions (losing the ‘E’ in EA). Externally, lifestyle drift could occur (as described in Joey's post) by giving up a substantial fraction of your effectively altruistic resources for non-effectively altruistic purposes, thus reducing your capacity to do good. Concretely, this could involve deciding to spend a lot of money on buying a (larger) house, having a (fancier) wedding, traveling around the world (more frequently or expensively), etc. Quoting myself:
Of course, changing your cause area or intervention to something that is equally or more effective within the EA framework does not count as value drift. Note that even if your future self were to decide to leave the EA community, as long as you still see ‘helping others effectively’ as one of your top-priorities in life it might not constitute value drift. (...)
Most of the potential value of EAs lies in the mid- to long-term, when more and more people in the community take up highly effective career paths and build their professional expertise to reach their ‘peak productivity’ (likely in their 40s). If value drift is common, then many of the people currently active in the community will cease to be interested in doing the most good long before they reach this point. This is why, speaking for myself, losing my altruistic motivation in the future would equal a small moral tragedy to my present self. I think that as EAs we can reasonably have a preference for our future selves not to abandon our fundamental commitment to altruism or effectiveness.